Memory is one type of integrated circuitry, and is used in computer systems for storing data. Integrated memory is usually fabricated in one or more arrays of individual memory cells. The memory cells might be volatile, semi-volatile, or nonvolatile. Nonvolatile memory cells can store data for extended periods of time, and in some instances can store data in the absence of power. Volatile memory dissipates and is therefore refreshed/rewritten to maintain data storage.
The memory cells are configured to retain or store memory in at least two different selectable states. In a binary system, the states are considered as either a “0” or a “1”. In other systems, at least some individual memory cells may be configured to store more than two levels or states of information.
Integrated circuitry fabrication continues to strive to produce smaller and denser integrated circuits. The smallest and simplest memory cell will likely be comprised of two electrically conductive electrodes having a programmable material received between them.
Programmable materials may also be referred to as memory cell materials. Suitable programmable materials have two or more selectable resistive states to enable storing of information by an individual memory cell. The reading of the cell comprises determination of which of the states the programmable material is in, and the writing of information to the cell comprises placing the programmable material in a predetermined resistive state. Some programmable materials retain a resistive state in the absence of refresh, and thus may be incorporated into nonvolatile memory cells.
Significant interest is presently being directed toward programmable materials that contain mobile charge carriers larger than electrons and holes. The charge carriers may be ions in some example applications. The programmable materials may be converted from one memory state to another by moving the mobile charge carriers therein to alter a distribution of charge density within the programmable materials. Some example memory devices that utilize ions as mobile charge carriers are resistive RAM (RRAM) cells; which can include classes of memory cells containing multivalent oxides, and which can include memristors in some specific applications. Other example memory devices that utilize ions as charge carriers are programmable metallization cells (PMCs); which may be alternatively referred to as a conductive bridging RAM (CBRAM), nanobridge memory, or electrolyte memory.
The RRAM cells may contain programmable material sandwiched between a pair of electrodes. The programming of the RRAM cells may comprise transitioning the programmable material between first a memory state in which charge density is relatively uniformly dispersed throughout the material and a second memory state in which the charge density is concentrated in a specific region of the material (for instance, a region closer to one electrode than the other). A PMC cell may similarly have programmable material sandwiched between a pair of current conductive electrodes. However, programming of the PMC comprises transitioning the PMC between a first state in which there is no conductive bridge between the electrodes and a second state in which mobile ions arrange to form super-ionic clusters or conducting filaments that electrically couple the electrodes to one another.
An ideal memory device will stably remain in a memory state after programming, and yet will be easy to program. It is difficult to accomplish both aspects of the ideal memory cell. Specifically, memory cells which are highly stable in their memory states also tend to be difficult to program (since it is difficult to induce the transition between the memory states during the programming operation); and conversely, memory cells which are easy to program tend to be unstable in their memory states.
It would be desirable to develop new memory cells which have the desired aspects of stability and relative ease of programming.